When to enable DNS Update (RFC2136) for DHCP-Server?


I’ve setup the DHCP-Server on the Green Network with Domain name suffix (e.g. office.local) and querying hosts with its office.local works as expected (even with connected OpenVPN Roadwarriors).

However I’ve read confusing reports concerning “Enable DNS Update (RFC2136)”:
some users said to turn it off so that domain name suffix works correctly…this seem confusing since I’ve setup myself a bind9/dhcp-server with rndc.key.

So should I turn ON or OFF “Enable DNS Update (RFC2136)” on ipfire to have DHCP-Server work correctly ? Strangely it works too if I turn it OFF…
So when is “Enable DNS Update” required ?

Is maybe “unbound” working differently than “bind9” (DNS-Server) ?